In cellular communication systems, resource allocation to wireless terminals on both downlink (communication from the network to the wireless terminal) and uplink (communication from the wireless terminal to the network) can be performed adaptively using scheduling. This scheduling can take into account instantaneous traffic pattern and radio propagation characteristics of each wireless terminal.
To facilitate efficient scheduling in the downlink, the wireless terminal sends rather small channel quality indicator (CQI) reports back to the radio base station of the network. These CQI reports are based on known downlink reference signals, which have been sent from the radio base station. In this way, effects of the radio propagation to the wireless terminal can be characterised and reported.
In the uplink, on the other hand, the radio base station can measure and estimate the channel directly, given that the radio base station has something to measure on. In LTE (Long Term Evolution), for example, a so called Sounding Reference Signal (SRS) in the uplink is provided as an uplink reference signal. Using an uplink reference signal, the uplink channel can be estimated. This allows the radio base station to schedule uplink transmission from terminals on frequencies with of good quality.
One issue is how the uplink reference signals are to be configured initially, before any radio bearers have been set up. On one hand, it is desired to send uplink reference signals for most or all frequency bands for uplink scheduling purposes. On the other hand, sending uplink reference signals puts requirements on the wireless terminal, increasing power and battery consumption.